This invention relates to a vacuum forming machine adapted to be arranged upstream of a foaming apparatus. The machine includes a conveying device for advancing a length portion of a thermoplastic web of indeterminate length or a thermoplastic plate of definite dimensions to a forming station and further comprises devices for firmly clamping such blanks in the forming station, heating devices for heating the blanks and a bridge which supports a positive mold such that it is vertically displaceable with respect to the bridge.
It is known to associate a vacuum forming machine with a foaming apparatus. In the forming station of the machine the article to be manufactured is deep-drawn and is subsequently filled with foam in a foaming apparatus. A number of components for the automotive industry are manufactured in this manner.
In German Utility Model (Gebrauchsmuster) No. 7,328,626 there is disclosed a vacuum forming machine which cooperates with a foaming apparatus. The negative deep-drawing molds are arranged for orbital motion, thus during operation they swing in sequence into the forming station. Such a known vacuum forming machine has been found to be disadvantageous in that a positive deformation of the blanks is not possible. Such a machine, however, has particular advantages in the deformation of grained sheets including grained foam sheets as well as the manufacture of relatively deep (large-height) components.
It is a further disadvantage of the vacuum forming machine disclosed in German Utility Model No. 7,328,626 that a relatively large number of negative deep-drawing molds has to be circulated. It is difficult to maintain all these molds at a predetermined temperature which is required for the deformation.
It is further known to deform the sheet blanks directly by the foam pressure; that is, the sheet blank is positioned in a cold state into the foaming mold. The pressure generated during foaming causes the sheet blank to conform to the mold wall. Such a method is feasible, however, only in case of flat and simple shapes. Otherwise, in case of grained sheets, the upper surface structure is distorted excessively and the resetting stresses in relatively deep articles having abrupt configurations lead to a warping of the article.